1. Field of the Invention
The present invention relates generally to nanometer-sized particles and in particular to nanometer-sized metal particles.
2. Description of the Background Art
Because of their size (1-100 nm) nanoparticles exhibit surface and volume effects not observed for larger dimension particles. As a result, it is believed that nanoparticles may have unique optical, dielectric, magnetic, mechanical, and transport properties which would find use in many types of applications such as catalysis, sensors, optics, ceramics and metallurgy. In order to be useful building blocks for new types of materials, it is desirable that nanoparticles have a high degree of purity, a narrow size distribution and chemical stability and should remain monodispersed during formation and processing into materials. The formation of well dispersed suspensions of nanoparticles is a particularly difficult challenge since particles of this size irreversibly agglomerate in order to minimize their high interfacial energy.
Conventionally, a number of approaches have been taken to synthesize nanoscale particles such as the vapor phase method, mechanical milling, and solution chemistry. Both vapor phase synthesis and mechanical milling produce agglomerated nanoparticles. Ceramic hydroxides, metals of group 6-11 elements, ferrofluids of amorphous iron as well as other nanoparticles have been synthesized using solution chemistry in the presence of surfactants in order to prevent agglomeration. Recently, some research efforts have been focussed on using vesicles as reaction cages to do solution chemistry with an aim to produce well-dispersed particles with a narrow size distribution. In addition, exterior vesicle surfaces have been decorated or coated with Ni, CdS, and ZnS. The vesicles coated on the exterior surface tend to agglomerate while particles formed with in vesicles remained dispersed. This approach suffers man, drawbacks such as disintegration of the vesicle, poor yield of the precipitated materials, lack of purity of the powders produced and difficulties in reproducibly processing these materials. In addition, the stability of the membranes to the chemical and physical stress that occurs during the synthesis and the processing of the nanoparticles demands a thorough investigation.